The present invention generally relates to superconductive magnets and, more particularly, is concerned with a superconductive magnet having a tube suspension assembly.
Superconductive magnets include superconductive coils which generate uniform and high strength magnetic fields, such as used, without limitation, in magnetic resonance imaging (MRI) systems employed in the field of medical diagnostics. The superconductive coils of the magnet typically are enclosed in a cryogenic vessel surrounded by a vacuum enclosure and insulated by a thermal shield interposed therebetween.
Various designs of tube suspension assemblies are employed to support the cryogenic vessel enclosing the superconductive coils of the magnet from and in spaced apart relation to both the thermal shield and the vacuum enclosure of the magnet. As one example, the tube suspension assembly can include overlapped fiberglass outer and inner support cylinders, such as disclosed in U.S. Pat. No. 5,530,413 to Minas et al. which is assigned to the same assignee as the present invention. In the Minas et al. tube suspension assembly, the outer support cylinder is located within and generally spaced apart from the vacuum enclosure and positioned outside of and generally spaced apart from the thermal shield. A first end of the outer support cylinder is rigidly connected to the vacuum enclosure while a second end of the outer support cylinder is rigidly connected to the thermal shield. The inner support cylinder is located within and generally spaced apart from the thermal shield and is positioned outside of and generally spaced apart from the cryogenic vessel. The inner support cylinder has a first end rigidly connected to the thermal shield near the second end of the outer support cylinder and has a second end located longitudinally between the first and second ends of the outer support cylinder and rigidly connected to the cryogenic vessel.
Problems can occur, however, with some designs of tube suspension assemblies at cryogenic temperatures. For instance, tube suspension assemblies of some superconductive magnet designs in MRI systems employ metal alloys or glass-epoxy materials. Metal alloys as well as glass-epoxy materials do not provide optimal load distributing and thermal insulating characteristics. Further, metal alloys are heavy and glass-epoxy materials deform as they tend to be compliant.
More recently, tube suspension assemblies are being developed that employ composite shells made of graphite-epoxy material and assembled together with step joint adhesive bonds to form the assembly. The graphite-epoxy material is stiffer than glass-epoxy material and tends to deform elastically rather than plastically. However, experiments and finite element analyses have shown that adhesively bonded joints will be under significant pealing when an axial load is applied and the step joint under significant rotation. Also tests have shown that adhesive bond strength varies significantly with the surface preparation and bond line thickness and other factors. The strength of the adhesive bond is part of the load path of the suspension system and thus its reliability is a limitation on the load limit of the whole suspension system.
Consequently, a need still exists for innovation with respect to superconductive magnet suspension assemblies which will provide a solution to the aforementioned problems.